1. Field of the Invention
The present invention relates to a toner and an electrophotographic image forming method using the toner.
2. Discussion of the Background
Various image forming methods using electrophotographic methods have been suggested. Developing methods applied to these image forming methods are broadly classified into dry developing methods and wet developing methods. The dry developing methods are further classified into one-component developing methods and two-component developing methods. A toner for use in either of the methods has to be positively or negatively charged in accordance with a polarity of an electrostatic latent image.
It is most effective to include a charge controlling agent to maintain the charge of a toner. Particularly, colorless or white charge controlling agents are indispensable for a color toner. Specific examples of the colorless or white charge controlling agents include metallic salt compounds of salicylic acid derivatives disclosed in Japanese Patent Publication No. 55-42752 and Japanese Laid-Open Patent Publications Nos. 61-69073, 61-221756 and 9-124659; and compounds of aromatic dicarboxylate metallic salts disclosed in Japanese Laid-Open Patent Publication No. 57-111541. However, these are chrome compounds which are harmful to the environment, or are not sufficiently colorless or white for the color toner.
In order to solve these problems, zirconium, which is a main quadrivalent metal, compounds with quadrivalent cations, bivalent cations which are oxo complexes and salicylic acids or their derivatives have been studied. These are colorless charge controlling agents having good dispersibility with resins and capable of imparting good chargeability to the toner. However, there still remains a problem of deterioration of the chargeability in an environment of high temperature and humidity.
Japanese Patents Nos. 3135507 and 3154088 disclose ratios among metallic compounds, inorganic ions and carboxylic derivatives. Japanese Laid-Open Patent Publication No. 2001-66830 discloses a constitutional unit of a zirconium atom and an aromatic carboxylic acid. These enabled the toner to maintain high frictional charge quantity even in the environment of high temperature and humidity. However, when a carrier has a low charge quantity, charge quantity distribution of a developer becomes broad and a problem of background development of the resultant image is not solved yet.
Japanese Laid-Open Patent Publication No. 2000-147828 stipulates X-ray diffraction of metallic salts of amorphous or low-crystallinity aromatic compounds. Although re-transfer is improved, structures of amorphous or low-crystallinity charge controlling agents are changed due to a heat or a shearing strength in a kneading process of producing the toner. Therefore, conditions of producing the toner is considerably restricted to obtain desired quality thereof or a developer deteriorates because the amorphous or low-crystallinity charge controlling agents contaminates a carrier.
Because charge controlling agents having satisfactory properties and qualities are not available, an image forming method providing good quality images having high transfer efficiency without background development due to changes of the environment and charge quantity of the carrier is not established yet.
Because a toner image and a transfer sheet directly contact with each other in a heat roller fixing method, the transfer sheet draws much heat energy. Surface temperature of the roller largely changes according to a copy mode, a kind of transfer sheet and the environment, and affects image qualities after fixed.
When the surface temperature of the roller is sufficiently high, fixability of a toner image does not particularly have a problem. However, a melted viscosity of a resin in a toner lowers and the toner image largely expands on the transfer sheet after the roller passes the toner image. Therefore, thin line reproducibility deteriorates and image quality is impaired. When the surface temperature of the roller is low, the melted viscosity of the resin in the toner is high and the toner image does not easily expand on the transfer sheet after the roller passes the toner image. However, the fixability of the toner deteriorates.
Several suggestions have been made to solve this problem. Japanese Patent No. 2743476 discloses a roll fixing toner including a resin-coated core particle formed from a polyester resin and a wax having a polar group, wherein the melted viscosities of the polyester resin and wax are specified.
Japanese Laid-Open Patent Publication No. 3-122661 and Japanese Patent Publication No. 8-16804 disclose a film fixing toner including a specific polyester resin and a release agent, wherein the melted viscosity of the polyester resin at from 80 to 120° C., graph gradients of the melted viscosity and temperature, and the melted viscosity of the release agent are specified. Japanese Patent Publication No. 8-12459 discloses a film fixing capsule toner formed of a specific polyester resin and a release agent, wherein the melted viscosity of the polyester resin at from 80 to 120° C., graph gradients of the melted viscosity and temperature, and the melted viscosity of the release agent are specified. Japanese Patent Publication No. 7-82250 discloses a film fixing toner including a specific polyester resin, an organic metallic compound and a release agent, wherein the melted viscosity of the polyester resin at from 120 to 150° C., graph gradients of the melted viscosity and temperature, and the melted viscosity of the release agent are specified. Japanese Patent Publication No. 7-72809 discloses a toner formed of a styrene-acrylic resin, herein a relationship between the melted viscosity and temperature at from 110 to 130° C. is specified. Japanese Laid-Open Patent Publication No. 10-246989 discloses a toner including a specific charge controlling agent, wherein the average viscosity gradient is specified.
However, although these conventional technologies have effects on improvements of fixability of the toner image, improvements of the image quality such as volume and area variations of the toner image are not considered.
In accordance with higher image quality, the toner particle diameter tends to become smaller. When the toner particle diameter is small, a pressure between the fixing rollers is not easily applied to the toner particles, and it is difficult to uniformly fix the toner image. Particularly, a fixer having a low surface pressure has such difficulty. In addition, a thin transfer sheet further decreases the surface pressure, and deteriorates the toner image fixability and the resultant image quality. To the contrary, a thick transfer sheet increases the surface pressure, which crushes the toner image to emphasize nonuniformity thereof and the resultant image quality deteriorates. This frequently occurs when digital development is used, and independent dot reproducibility is largely affected. Therefore, a heat roller fixing method having higher heat efficiency than the other heat fixing methods is widely used, which includes one or two roller having an elastic layer.
Halftone image density has to be uniform, and microscopic unevenness thereof gives the impression of a rough image. The roughness is physically evaluated by granularity.
A noise of the image can be measured by Wiener Spectrum (WS) which is a frequency characteristic of the image density variation.
When the image density variation having an average value of 0 is f (x)F(u)=∫f(x)exp(−2πiux)dx  formula 1WS(u)=F(u)2  formula 2                wherein, u is a spatial frequency.        
The granularity is an integral value of the WS and Visual Transfer Function (VTF), and can be determined by the following formula:GS=exp(−1.8<D>)˜WS(u)1/2VTF(u)du  formula 3                wherein, exp(−1.8<D>) is a correction coefficient between the density and a perceptive brightness by a human, and <D> is an average value of the density.        
The granularity has a high correlation with a subjective evaluation of the image smoothness. The smaller the granularity, the smoother and better the image quality. To the contrary, the larger the granularity, the rougher and worse the image quality.
Because of these reasons, a need exists for a toner and an image forming method having high transferability and producing good images even in an environment of high temperature and humidity and when a carrier is insufficiently charged.